The present invention relates to a process for partially hydrogenating monocyclic aromatic hydrocarbons to produce corresponding cycloolefins,in particular, cyclohexenes in high selectivity and yield.
Cyclohexenes are of high commercial value for use as intermediates for the manufacture of organic chemical engineering products and are particularly important for use as materials for the production of polyamides and lysines. Several methods have been proposed for producing cyclohexenes by reacting aromatic hydrocarbons with hydrogen, and they include: (1) a method using water, an alkali agent, and a catalyst composition containing a reduced cation of group VIII in the periodic table (Japanese Patent Publication No. 22850/1981 and U.S. Pat. No. 3,767,720); (2) a method using a ruthenium catalyst, supported on an oxide of nickel, cobalt, chromium, titanium or zirconium, with an alcohol or an ester being used as an additive (Japanese Patent Publication No. 3933/1977); (3) a method using a Cu, Ag, Co or K-containing ruthenium catalyst, water and a phosphate salt compound, or a method using a catalyst having ruthenium supported on a carrier such as alumina, in combination with water and a phosphate salt compound of cobalt, nickel or copper (Japanese Patent Publication Nos. 4536/1981 and 35646/1981, and U.S. Pat. No. 4,197,415); (4) a method wherein reaction is carried out in the presence of a neutral or acidic aqueous solution containing a ruthenium catalyst and a salt of at least one cation selected from the group consisting of metals of groups IA and IIA in the periodic table and manganese (Japanese Patent Publication No. 7607/1982); (5) a method wherein reaction is carried out in the presence of water using a solid catalyst that is based on at least one of ruthenium and rhodium and which has been preliminarily treated with an aqueous solution containing a salt of at least one cation selected from the group consisting of metals of groups IA and IIA in the periodic table, manganese, iron and zinc (Japanese Laid-open Patent Application No. 98243/1976 and USP 4,055,512); (6) a method wherein reaction is carried out in the presence of a ruthenium catalyst, with at least one of zinc oxide and zinc hydroxide being added to the reaction system as an activator component [Japanese Laid-open Patent Application No. 184138/1984]; and (7) a method using metallic ruthenium crystallites having an average crystallite size of 200 .ANG. or less and/or agglomerated grains thereof in the presence of water and at least one zinc compound [Japanese Laid-open Patent Application No. 50930/1986].
However, in order to enhance the selectivity for the desired cyclohexenes in these prior art methods, the conversion of the monocyclic aromatic hydrocarbons used as starting materials must be considerably reduced and, in addition, the rate of reaction that can be achieved in these methods is very limited. Therefore, the yield and rate of production of cyclohexenes that can be attained in these methods are too low to justify their implementation on a commercial scale.
In order to develop commercial processes for the production of cyclohexenes, it is essential and important that the catalyst used in the reaction should maintain high activity and selectivity in continued operation, but none of the prior art methods have been completely satisfactory in this respect.
According to the studies conducted by the present inventors, if metallic ruthenium crystallites and/or agglomerated grains thereof is used as the sole catalyst as proposed in Japanese Laid-open Patent Application No. 50930/1986, cycloolefins can sometimes be obtained in a comparatively high yield but, on the other hand, difficulty is frequently encountered in maintaining a stable reaction system because of such phenomena as adhesion or deposition of the catalyst on the area of contact between the reactor and the reaction solution and the deterioration of the catalyst per se.